Thermal analysis (TA) like thermogravimetry (TG) represents an important sector of instrumental analysis for example in chemistry and industrial process controlling. TA is utilized to characterize temperature dependent material properties, to evaluate thermo dynamical conversions and/or thermophysical parameters, as well as to observe chemical reactions. But for more detailed and advanced applications often a chemical investigation of the evolved gases—evolved gas analysis (EGA)—is indispensable. Depending on the requirements, this EGA can be realized with sequentially working analytical devices like gas chromatographs (GC) or with on-line real-time analytical devices, e.g. mass spectrometers (MS). These approaches enable the investigation of chemical classes and species of the evolved molecules. The mass spectrometry method commonly used in conjunction with TG is quadrupole mass spectrometry (QMS) with electron ionization (EI). Generally, the currently available MS systems use electrons with a kinetic energy of 70 eV for electron impact ionization. Typically, EI mass spectra hardly contain molecular ion signals beside the dominating fragment peaks due to the excess energy of these electrons. These fragment signals are characteristic for different molecules and can be used for alignment as a so called finger print of species in conjunction with EI data bases in order to identify molecules and its chemical structure.
However, in case of samples like polymers, plastic blends, plastic copolymers and fossil fuels containing multiple organic compounds, fragment peaks overlap and complicate or even prevent identification of evolving original substances from the TG system. To overcome this problem, different approaches are known to the applicant and have been developed by several research groups in the past decades. One auspicious method is to couple TA with a separation technique, e.g. GC in combination with MS for EGA. Due to the separation technique, the analytes disperse along the GC-capillary and reach the ion source of the MS at different retention times. Consequently the number of simultaneously ionized compounds is reduced and therefore the EI caused fragments do not hinder identification. In case of Pyrolysis-GC-MS (Py-GC-MS), samples are pyrolyzed instantly and a narrow plug is transferred to the GC-MS, where it can be separated within only one GC run. This method however does not provide TA relevant signals, e.g. mass loss or information about the enthalpy of formation of the relevant sample. Furthermore, the reaction kinetics can be totally different from a continuously heated analysis of the sample. However, TA-GC-MS and temperature controlled Py-GC-MS are comparable due to its opportunities regarding the analysis of pyrolysis products in many cases. Due to the offline character of the GC technique, the assay is usually collected offline and analyzed separately with the GC-MS. This is associated with sample preparation and longer measurement periods and additionally, chemical reactions between products are not to be excluded upon subsequent sample handling (heating).
An online coupling of the TG-GC-MS can be realized using valve systems in combination with sample loops allowing a quasi continuous operation of the TG-GC-MS system. Although these systems can uniquely be used for high performance qualitative and quantitative EGA, which is often based on prior knowledge of the sample, only parts of the molecular composition are usually analyzed, while sample information during two GC runs get lost (heart cut technique).
Another promising way is using soft ionization methods, where fragmentation of organic molecules from EGA can be circumvented. MS with soft ionization provide mass spectra containing basically molecular ion peaks [M+] which are therefore clear and easy to interpret. First thermal decomposition measurements and pyrolysis studies using laser based photon ionization revealed that highly valuable molecular information on the thermal decomposition processes can be achieved. Since laser based instrumentation often involves big drawbacks, regarding the high costs and complexity of the laser devices, vacuum ultraviolet (VUV) lamps such as deuterium or krypton discharge and electron beam pumped excimer Lamps (EBEL) are an attractive option. In case of single photon ionization (SPI), only one photon with the energy Ehu higher than the ionization energy EI of the relevant molecule can lead to ionization. Consequently, the photon energy determines selectivity and acts as an energetic threshold. Molecules with an EI less than Ehu can be ionized without any regard to its structure and formation in contrast to resonance enhanced multi photon ionization (REMPI). Although SPI mass spectra mainly consist of molecular ion peaks, a correlation between mass information and molecular structure is impossible, if there is no information about the sample composition. Therefore, SPI as a quasi selective ionization method is not appropriate to differ between isobaric and isomeric molecules.
To improve the resolving power and the isobaric separation ability of an analytical device and therefore to achieve a clearer conclusion about a sample composition, two or more independent separating methods can be coupled inline. One of the widespread multi dimensional analysis (MDA) techniques is multi dimensional separation using two dimensional GC (GC−GC or GC×GC), where GC−GC represents the so called heart cut technique. Two different chromatographic capillaries are connected serially. Often valve systems are utilized between the two columns to extract fractions of interest and lead this fractions to the second column for further separation. GC×GC, which is a comprehensive hyphenation in contrast to GC−GC, uses modulators and a short second GC capillary (fast GC). The modulator acts as a device connected between the first and the second GC-column to focus and to re-inject the eluents from the first into the second column. The choice of the type and length of the second column as well as the modulator type and modulation time is thereby crucial for the best achievable separation power. Single photon ionization mass spectroscopy (SPIMS) with its soft ionization character acts also as a separation technique. Therefore GCxGC in its essential principles closely resembles GC×SPIMS. In both, the samples components are dispersed in time by the GC and are lead to the secondary analytical device either individually or at least in greatly simplified sub-mixtures. Where the first GC creates a primary retention time axis, the second instrument with its own resolving power operates as a detector for the inlet GC and provides an independent analysis of the dispersed sample eluting from the primary GC. GC×GC as well as GC×SPIMS combine independent analytical techniques and generate comprehensive two dimensional data sets. In case of GC×GC, the two dimensions are given by the two different retention time axes and each substance is defined by its specific location in the separation plane. Sample constituents investigated with GC×SPIMS are also defined by one specific location in its dimensions, which is determined by its retention time in the first and the corresponding molecular mass to charge ratio of each component (m/z) in the second. For more detailed analysis further methods can be hyphenated to each other. With each additional application, the selectivity of the system will increase and the data set extends by one more dimensions. Beside the increased resolution power of the common analytical device, it is a challenge to keep the complexity of the apparatus low and to provide, that after each separation, the entirety of the eluents is lead to the next step without merging.